Manufacture of tennis strings



Aug. 21, 1962 J. T. CRANDALL 3,050,431

MANUFACTURE OF TENNIS STRINGS Filed July 7, 1958 INVENTOR.

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United States Patent ()fi ice Patented Aug. 21, 1952 3,050,431 MANUFACTURE OF TENNIS SGS Julian T. Crandall, Ashaway, R.I., assignor to Ashaway Line & Twine Mfg. Co., Ashaway, R.I., a corporation of Rhode Island Filed July 7, 1958, Ser. No. 747,063 2 Claims. (Cl. 156-172) The present invention relates generally to the manufacture of strings for athletic rackets, such as tennis, badminton, squash, and the like, as well as for musical instruments, and more specifically is concerned with the provision of an improved method for manufacturing strings of this general type, which method involves the use of a novel forming and heating die.

A primary object of the instant invention is the provision of a method and technique which is of particular value in connection with the manufacture of plastic strings comprising a core having an armor sheath thereabout.

An important object of my invention is the provision of a method of manufacture utilizing a novel forming and heating die, which functions to provide a synthetic string of the type above described which is highly and effectively integrated.

Another important object of this invention is the provision of a method of manufacture which results in a plastic string having a high degree of strength for a relatively small diameter.

A further object of the present invention is the provision of a method of the character described wherein it is not necessary to dip the string in any sort of solution or the like after the armor sheath has been applied to the core, nor need the core be soft or tacky when the sheath is applied thereto.

Still another object of my invention is the provision of a method of manufacture which is not unduly expensive or complicated to perform and which results in a well integrated, rugged and durable string, as Well as one having the desired degree of resilience.

Other objects, features and advantages of the invention will become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.

In the drawings which illustrate the best mode presently contemplated by me for carrying out my invention:

FIGS. 1 through 4 illustrate the successive steps in the formation of one type of central core which may be utilized in the instant invention;

FIG. 5 is a schematic or diagrammatic illustration of the various steps which comprise the instant method;

FIG. 6 is a fragmentary elevational view, partly in section, showing the string passing through the die which forms a part of the instant invention;

FIG. 7 is a side elevational view of the die per se; and

FIG. 8 is a bottom view thereof.

In the manufacture of synthetic strings for tennis rackets and the like, it has heretofore been common practice to provide a composite string having a thermoplastic core surrounded by a thermoplastic armor sheath, either braid or spiral wound, and then integrating th entire string by various means. In virtually all cases, it has been found that to acquire proper integration of a composite string of this type it is necessary to pass the string through a plastic solution subsequent to the application of the sheath to the core, or at the very least, to apply the sheath to the core while the latter is still soft and tacky. Now for the first time a method of manufacture has been developed whereby a string of this type may be integrated and bonded even more effectively than the prior art constructions and without the necessity of utilizing the aforementioned dip or tacky core. Thus, by the method and apparatus of the instant invention, it is possible to provide an extremely well-bonded and integrated string of the character described whereby the string possesses great strength, resilience, resistance to tension strains, etc., while at the same time maintaining the diameter of the string at a lesser value than has heretofore been possible where the strings were manufactured by the prior art methods.

The basic concept of the instant invention is the provision of a specially designed forming and heating die which receives the string as, soon as the outer sheath is applied to the core, said die functioning first of all to insure that the armor sheath is properly formed and positioned over the core and then functioning to eifectively bond and integrate the core and sheath to provide a string having the desirable characteristics enumerated supra. Thus, in accordance with the invention now to be described, it is possible to utilize a core which need not be soft or tacky when the armor sheath is applied thereabout, but rather the entire forming and integration of the string is accomplished by my novel and inventive die and without the necessity of any subsequent dips or the like, which not only render the manufacturing procedure more complicated and expensive, but which also cause the finished string to have a greater diameter than necessary, thereby lessening the desired resilience of same.

Referring now to the drawings, FIGS. 1 through 4 illustrate one form of core construction which may be utilized in connection with the instant invention. Thus, the core '10 is illustrated as being of the multi-filament type, the filaments 12 preferably being of a thermoplastic, such as nylon. In the illustrated form, the filaments 12 are cabled in groups of three to provide a bundle 14, and then three of said bundles are cabled or gently twisted to provide a multifilament strand 16 having nine filaments. The strand 16 is then passed through a tank containing a solution of a special coatiu g material, which preferably has a nylon base, it being preferred to apply several thin layers of coating rather than a single heavier layer, and in practice it has been found that sixteen dips provide the desired amount of coating and bonding material 18. After the core has been dipped in the manner afore indicated, it is oriented under heat and pressure to provide the finished core 10 having a relatively smooth outer surface as illustrated in FIG. 4. It should be understood that the construction of the core actually forms no part of the instant invention and that a multifilament core having any desired number of filaments and integrated in any suitable manner may be used. Also, it is not essential that the filament be twisted or cabled as illustrated in FIGS. 1 and 2, but rather they may be positioned in straight parallel alignment, although it has been found that twisting or cabling of the filaments not only facilitates the manufacture of the core, but also results in a somewhat stronger and more flexible arrangement.

While it has been found that a multifilament core is preferable for reasons of strength and flexibility, in some cases it may be desirable to use a monofilament core, and

such may be used without departing from the spirit of my invention. At any rate, the core 10, whether multifilament as illustrated or monofilament, is fed from a supply reel '20 around idler 22 to a station generally indicated at 24 where an armor sheath 26 is applied thereto by a conventional wrapping machine (not shown). In FIG. it will be noted that the wrapping machine is illustrated as having four bobbins 28 which apply a spiral wrap to the core in a well-known and conventional manner, note FIG. 6. Here again, it will be understood that the simultaneous application of four ends 26 to the core 10 is merely illustrative, and in actual practice any desired number of bobbins 28 may be utilized. Also, although the invention is illustrated in connection with a spiral wrap, it is also within the realm of this invention to apply a braided sheath if such is desired. \Nhether spiral or braided, however, the armor sheath 26 may be either of multifilament or monofilament construction.

Since the core 10 and the sheath 26 are in no way softened when the latter is applied to the former, I utilize a forming and heating die, generally indicated at 30, to form and integrate the composite string. As will be seen most clearly in FIGS. 6 through 8, die 30 comprises a metallic, generally rectangular-shaped housing 32 having a passageway or bore 34 extending therethrough. A first die plate 36 is secured 'at the entrance edge of passageway 34 by any desirable means, such as screw 38, it being understood that housing 32 is provided with a recess 40 which snugly receives the said die plate. Die plate 36 comprises an orifice 42 extending therethrough in alignment with passageway 34, said orifice having a bevel 44 at its leading edge. At the opposite extremity of housing 32 and passageway 34 there is provided a second die plate 46 of similar construction to die plate 36 and having an orifice 48 extending therethrough beveled at its inner edge as at 50. It is important to note that exit orifice 48 is of a diameter which is 5 to 10 percent smaller than the diameter of entrance orifice 42, it being understood that the actual sizes of said orifices will be determined by the size of the string being manufactured. More specifically, entrance orifice 42 will be of a size just large enough to snugly receive the string therethrough after the armor sheath 26 has been wound thereon, while exit orifice 48 is somewhat smaller in order to effectively and properly integrate the string as it exits from the die 30. In practice, it has been found that a workable and effective tennis string may be manufactured by using an entrance orifice of .060 inch in diameter and an exit orifice of .056 inch in diameter.

Die 30- carries a suitable heating element, such as an electric cartridge heater 52 and a thermoswitch 54, whereby the die, and primarily passageway 34, may be maintained at a regulated and desired temperature. Suitable insulation (not shown) may be provided around the die 30 so as to maintain the generated heat therein. The diameter of passageway 34 is not critical, although it is important that the passageway be sufficiently large in cross section so as to enable the composite string 56 to pass therethrough without danger of the string brushing against the sides of said passageway. The composite string 56 is drawn through die 32 by means of a take-up roll 58 which takes up the string over a suitable idler 60. The rate at which the string is drawn through die 32 is determined by the size of the string being formed, it having been found that a slower rate of speed is preferable for smaller sized strings, and a relatively faster rate of speed for larger strings. Conversely, the rate at which the string is drawn through die 32 determines the heat at which die 30 is maintained. Thus, the faster the rate of speed, the greater the heat required; but it has generally been found that for the best results the string should be drawn at a rate of 7 /2 to 10 inches per minute, and the die maintained at a heat of from 330 F. to 360 F. The easy removability of die plates 36 and 46 enables die 30 to be readily converted for the manufacture of different size strings. As will be obvious from FIG. 6, it has been found that best integration of the string is obtained where the length of the passages defined by each orifice is approximately twice the orifice diameter and where the latter is approximately one-half the bore diameter.

In operation and use, die 30 is positioned so that entrance orifice 42 receives string 56 immediately upon application of the armor sheath 26, as clearly illustrated in FIG. 6. As hereinbefore indicated, the entrance die functions to smooth and properly form the armor sheath on the core 10, after which the heated area or passageway 34 softens the sheath sufficiently so that as it is drawn through the smaller die orifice 48 the softened plastic is embedded to the core to provide a well-integrated string. Actually, it has been found that as the string is drawn through the smaller die orifice 48, a small amount of the soft plastic is scraped from the outer surface of the string and positions itself within the beveled area 50 whereupon there is always a reservoir of soft plastic which will be forced into any voids or openings that exist in the string to insure that a well-bonded, integrated and solid string is provided. Once the string leaves the die 30, no further treatment or dips of any sort are necessary since the slow rate at which the string is traveling enables it to cool and set before reaching idler 60 and take-up reel 58.

If it is desired to provide a double spiral wrap, usually of opposite hand, to core 10, then the first wrap must be applied in the manner afore indicated and the wrapped string passed through die 30 before the second wrap is applied. Preferably, after the string with the first wrap has been drawn through the die, it is dipped in a plastic solution before application of the second wrap. Then after the second wrap is applied the completely wrapped string must once again be passed through the heating and forming die.

Although it is not necessary to utilize any sort of initial dip in a thermoplastic solution, solvent, or the like before the string first reaches die 30, it should be emphasized that such a dip may be utilized if desired either before the armor sheath is applied to the core, after it is applied, or both. Thus, while use of my novel and effective die 30 enables these preliminary dips to be dispensed with, while at the same time enabling a completely set and solidified armor sheath and core to be utilized, it will be understood that these preliminary dips still can be used, if desired, and my die 30 will still function to provide a better integrated and bonded string.

Actually, it has been found that where the core 10 is a monofilament, die 30 will not soften the core sufficiently to enable effective integration of the composite stiring to be accomplished, and hence where the core. is a monofilament, it is preferable to dip. same before the. armor sheath is applied. The exact formulation of the dip which may be utilized in such a situation forms no: part of this invention, although such formulation is completely described in my issued patents, Nos. 2,712,263 and 2,735,258, each relating to this same general type of sub ject matter.

While there is shown and described herein certain speoific steps and structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular method and forms herein shown and described except in so far as indicated by the scope of the appended claims.

I claim:

1. The method of manufacturing an integrated plastic string of the character described, comprising the following steps:

(A) taking a flexible thermoplastic core;

(B) winding a thermoplastic strand around said core to provide an armor sheath thereover; and

(C) drawing the composite core and sheath in a dry state through a heated die having an onifice of lesser diameter than that of the composite string, said string being integrated upon removal from the die.

2. The method of manufacturing an integrated plastic string of the character described, comprising the following steps:

(A) taking a flexible thermoplastic core;

(B) Winding a thermoplastic strand around said core to provide an armor sheath thereover;

(C) subjecting the composite string in its dry state to heat;

(D) drawing said composite string through a first die orifice having a diameter substantially equal to that of the composite string; and

References Cited in the file of this patent UNITED STATES PATENTS Francis Mar. 9, 1943 Francis July 8, 1952 Boone Aug. 31, 1954 MacHenry Nov. 12, 1957 Hudak Dec. 17, 1957 

1. THE METHOD OF MANUFACTURING AN INTEGRATED PLASTIC STRING OF THE CHARACTER DESCRIBED, COMPRISING THE FOLLOWING STEPS: (A) TAKING A FLEXIBLE THERMOPLASTIC CORE; (B) WINDING A THERMOPLASTIC STRAND AROUND SAID CORE TO PROVIDE AN ARMOR SHEATH THEREOVER; AND (C) DRAWING THE COMPOSITE CORE AND SHEATH IN A DRY STATE THROUGH A HEATED DIE HAVING AN ORIFICE OF LESSER DIAMETER THAN THAT OF THE COMPOSITE STRING, SAID STRING BEING INTEGRATED UPON REMOVAL FROM THE DIE. 